1. Field of the Invention
This invention relates generally to the field of seismic gas exploders and more particularly to an improved method and apparatus for charging such devices with an explosive mixture in the desired proportion.
2. Description of the Prior Art
There are innumerable uses for gases which are combined in various predetermined ratios, wherein the efficiency of such use is highly dependent upon the accuracy and repeatability with which such ratios can be established and maintained. One such use is the field of seismic gas exploders, particularly marine, for generation of acoustic pulses, wherein oxygen and a fuel gas, such as propane, are introduced into a combustion chamber through variable size orifices. In operation, ignition of the gas mixture propels a plate through the water to create an acoustic disturbance.
In this prior art system, relatively large volume gas bottles or containers are continuously interconnected with individual surge chambers which are regulated to a pressure well in excess of that in the mixture to be combusted. When a fill operation is undertaken, solenoid operated valves are opened between the surge chambers and the orifices after passing through check valves. The fill continues for a fixed period of time, the percentage of each gas in the resultant mixture being determined by the orifice parameters provided all gas pressures are fixed.
The difficulty with this arrangement is that the integrity of very smll orifices, for example, on the order of several 64ths of an inch, is hard to maintain. This is because of the presence of contaminants in the orifice such as carbon chips or oil and/or the existence of mechanical problems such as surface irregularities.
In the above device, the accuracy of filling operations depends upon so-called orifice equations, which take account of factors such as friction, gas compressibility, orifice size and shape, the presence of laminar or turbulent flow, and the like. These equations cannot predict the character of the resultant gas mixture with reliability. Yet, in field operations involving marine seismic gas exploders, mixture control is important in optimizing the time of occurrence and amplitude of the predominant peak of the acoustic wave resulting from a shot. With presently available methods, the gas mixtures introduced into a marine gun must be adjusted on an individual basis, (i.e., by varying the orifice fittings) which requires that the gun be removed from the water and put out of service. It would be more efficient and otherwise desirable to be able to adjust the gas mixtures of a series of deployed marine exploders as a group without interruption of operations or removal of the devices from the water.
A further problem with prior art mixing systems for such marine gas exploders is that gases introduced individually into a combustion chamber, even in the desired ratios, may not be homogeneously mixed within the chamber at the time of detonation, if such detonation is programmed to occur after a very short time delay.
It is, therefore, a general object of this invention to provide a method and apparatus for charging combustion gases into a seismic gas exploder in a desired ratio with a high degree of accuracy and repeatability.
It is a further object of this invention to provide a method and apparatus for charging the combustion chamber of a marine seismic gas exploder with two or more gases to a desired ratio wherein such ratio may be readily varied.
These and other objects and advantages of this invention will become apparent from a consideration of the detailed description and drawings to follow.